app.js

var express = require('express'),
		api			= express();

var validateVersion = function (matched, def, callback) {
	var header = null;
	if (typeof def === 'function') {
		callback = def;
		def = false;
	}
	return function (req, res, next) {
		if (req.header('X-Transistor-Version')) {
			header = req.header('X-Transistor-Version');
		}
		if (header && matched === header) {
			return callback(req, res, next);
		} else if (!header && def === true) {
			return callback(req, res, next);
		}
		return next();
	}
}

api.get('/podcasts',
	validateVersion('2', function(req, res) {
		res.send('Hello from Version 2');
	})
);

api.get('/podcasts',
	validateVersion('1', true, function (req, res) {
		res.send('Hello from Version 1');
	})
);

api.listen(3000);